We propose to use IVET to investigate the interaction between P. syringae pv. maculicola race M6 (Psm M6) and Arabidopsis. Psm M6 is compatible on ecotypes of Arabidopsis lacking Rpm1. However, it carries avrRpm1 and is incompatible on Arabidopsis expressing Rpm1. A small percentage of Psm M6 cells recovered after growth in planta are virulent on all tested ecotypes of Arabidopsis. Rearrangements in the genome, resulting in a Tn3 insertion into avrRpm1 and an excision of a plasmid, were demonstrated to have occurred in these now virulent cells. This is an active response by Psm M6 to signals of the plant. We are interested in characterizing the induced genes of Psm M6 for initiating disease in compatible Arabidopsis plants and the genes that respond to in planta signals which lead to the plasmid excision. These studies will provide a comprehensive analysis of how Psm M6 initiates disease in Arabidopsis. IVET is a promoter-trapping technique that has been used successfully to identify induced virulence genes of many pathogenic bacteria. IVET will be adapted to increase its capability, reduce the amount of labor required, and examine only the initial events of pathogenesis. All necessary DNA constructs are in our possession and necessary modifications are facile and in progress. We have already demonstrated that several facets of the proposal are feasible. Finally, we present several possible directions to pursue in characterizing the mechanisms of pathogenesis. A complete understanding could lead to the creation of novel, more robust forms of resistance against phytopathogenic bacteria.